Manufacturing Processes lab I Cutting tools

CUTTING TOOL TECHNOLOGY Tool Life Tool Materials Tool Geometry Twist Drills

Three Modes of Tool Failure Fracture failure: Cutting force becomes excessive and/or dynamic, leading to brittle fracture Temperature failure: Cutting temperature is too high for the tool material Gradual wear: Gradual wearing of the cutting tool (leads to the longest possible use of the tool )

Tool Materials Tool failure modes identify the important properties that a tool material should possess: Toughness ‑ to avoid fracture failure Hot hardness ‑ ability to retain hardness at high temperatures Wear resistance ‑ hardness is the most important property to resist abrasive wear

Tool Materials Tools are made of: High Speed Steel (HSS) Cemented carbides Non‑steel Cutting Carbide Grades Steel Cutting Carbide Grades Cermets Coated Carbides Ceramics Synthetic Diamonds Cubic Boron Nitride

High Speed Steel (HSS) Highly alloyed tool steel capable of maintaining hardness at elevated temperatures (better than high carbon and low alloy steels) One of the most important cutting tool materials Especially suited to applications involving complicated tool geometries, such as drills, taps, milling cutters Two basic types (AISI or American Iron and Steel Institute) Tungsten‑type, designated T‑ grades Molybdenum‑type, designated M‑grades

Cemented Carbides Class of hard tool material based on tungsten carbide using powder metallurgy techniques with cobalt (Co) as the binder. Two basic types: Non‑steel cutting grades (Used for nonferrous metals and gray cast iron) Steel cutting grades (Used for low carbon, stainless, and other alloy steels)

Cemented Carbides – General Properties High compressive strength but low‑to‑moderate tensile strength High hardness Good hot hardness Good wear resistance High thermal conductivity High elastic modulus ‑ 600 x 103 MPa (90 x 106 lb/in2) Toughness lower than high speed steel

Cermets Bonded material containing ceramics and metals, widely used in jet engines and nuclear reactors. Cermets behave much like metals but have the great heat resistance of ceramics. Tungsten carbide, titanium, zirconium bromide, and aluminum oxide are among the ceramics used; iron, cobalt, nickel, and chromium are among the metals. Properties: Higher speeds and lower feeds than steel‑cutting carbide grades. Better finish achieved, often eliminating need for grinding. Applications: high speed finishing and semifinishing of steels, stainless steels, and cast irons

Coated Carbides Cemented carbide insert coated with one or more thin layers of wear resistant materials, such as TiC, TiN, and/or Al2O3 Coating applied by chemical vapor deposition or physical vapor deposition. Coating thickness = 2.5 ‑ 13 m (0.0001 to 0.0005 in). Applications: cast irons and steels in turning and milling operations. Best applied at high speeds where dynamic force and thermal shock are minimal.

Coated Carbide Tool Photomicrograph of cross section of multiple coatings on cemented carbide tool (photo courtesy of Kennametal Inc.)

Ceramics Primarily fine‑grained Al2O3, pressed and sintered at high pressures and temperatures into insert form with no binder. Applications: high speed turning of cast iron and steel.

Synthetic Diamonds Sintered polycrystalline diamond (SPD) - fabricated by sintering very fine‑grained diamond crystals under high temperatures and pressures into desired shape with little or no binder. Applications: high speed machining of nonferrous metals and abrasive nonmetals such as fiberglass, graphite, and wood Not for steel cutting.

Cubic Boron Nitride Next to diamond, cubic boron nitride (cBN) is hardest material known. Fabrication into cutting tool inserts same as SPD, or used as coatings. Applications: machining steel and nickel‑based alloys SPD and cBN tools are expensive.

Tool Geometry Two categories: Single point tools: Used for turning, boring, shaping. Multiple cutting edge tools: Used for drilling, reaming, tapping, milling, broaching, and sawing.

Single-Point Tool Geometry Figure 23.7 (a) Seven elements of single‑point tool geometry; and (b) the tool signature convention that defines the seven elements.

Holding the Tool Figure 23.9 Three ways of holding and presenting the cutting edge for a single‑point tool: (a) solid tool, typical of HSS; (b) brazed insert, one way of holding a cemented carbide insert; and (c) mechanically clamped insert, used for cemented carbides, ceramics, and other very hard tool materials.

Common Insert Shapes Figure 23.10 Common insert shapes: (a) round, (b) square, (c) rhombus with two 80 point angles, (d) hexagon with three 80 point angles, (e) triangle (equilateral), (f) rhombus with two 55 point angles, (g) rhombus with two 35 point angles. Also shown are typical features of the geometry.

A collection of metal cutting inserts made of various materials (photo courtesy of Kennametal Inc.).

Twist Drills By far the most common cutting tools for hole‑making Usually made of high speed steel Figure 23.12 Standard geometry of a twist drill.

Twist Drill Operation Rotation and feeding of drill bit result in relative motion between cutting edges and workpiece to form the chips Cutting speed varies along cutting edges as a function of distance from axis of rotation Relative velocity at drill point is zero, so no cutting takes place A large thrust force is required to drive the drill forward into hole

Twist Drill Operation - Problems Chip removal Flutes must provide sufficient clearance to allow chips to be extracted from bottom of hole during the cutting operation Friction makes matters worse Rubbing between outside diameter of drill bit and newly formed hole Delivery of cutting fluid to drill point to reduce friction and heat is difficult because chips are flowing in opposite direction